Hot melt thermoplastic adhesives have been widely used in industry for adhering many types of products, and are particularly useful in applications where quick setting time is advantageous. One application for hot melt adhesive which as been of considerable interest in recent years is the bonding of non-woven fibrous material to a polyurethane substrate in articles such as disposable diapers, incontinence pads and similar articles.
One aspect of forming an appropriate bond between the non-woven layer and polyurethane substrate of a disposable diaper, for example, is to avoid loss of adhesive in the valley or gaps formed in the irregular surface of the chopped fibrous or fluff-type material which forms the non-woven layer. If the adhesive is discharged onto the non-woven layer in droplet form, for example, a portion of the droplets can fall between the gaps in the surface of the fibrous, non-woven material. As a result, additional quantities of adhesive are required to obtain the desired bond strength between the polyurethane substrate and non-woven material.
This problem has been overcome in the prior art by forming hot melt thermoplastic adhesives in elongated, thin beads or fibers which are deposited atop the non-woven material and span the gaps in its irregular surface. Elongated beads or fibers of adhesive have been produced in prior art spray devices which include a nozzle formed with an adhesive discharge opening and one or more air jet orifices through which a jet of air is ejected. A bead of adhesive is ejected from the adhesive discharge opening in the nozzle which is then impinged by the air jets to attenuate or stretch the adhesive bead forming a thin fiber for deposition onto the substrate. Examples of spray devices of this type are disclosed in U.S. Pat. Nos. 2,626,424 to Hawthorne, Jr.; 3,152,923 to Marshall et al; and, 4,185,981 to Ohsato et al.
In applications such as the formation of disposable diapers, it is important to carefully control the spray pattern of adhesive fibers deposited onto the non-woven substrate in order to obtain the desired bond strength between the non-woven layer and polyurethane substrates using as little adhesive as possible. Improved control of the spray pattern of adhesive fibers have been obtained in prior art spray devices of the type described above by impacting the adhesive bead discharged from the nozzle with air jets directed substantially tangent to the adhesive bead. The tangentially applied air jets control the motion of the elongated fibers of adhesive formed from the adhesive bead ejected from the adhesive discharge opening in the gun nozzle, and confine the elongated fibers in a relatively tight, or compact, spiral pattern for application onto the substrate. Structure which produces a spiral spray pattern of adhesive fibers for deposition onto a substrate is disclosed, for example, in Hawthorne, Jr. U.S. Pat. No. 2,626,424 and Ohsato et al U.S. Pat. No. 3,152,923 mentioned above.
In order to produce a compact spiral spray pattern of adhesive fibers in the spray devices described above, it is important to ensure that the air jets are accurately directed tangent to the bead of adhesive ejected from the nozzle of the spray device. This requires accurate placement of the bores or passageways through which pressurized air is ejected to form the air jets in the nozzle or gun body of the spray device, which are typically on the order of about 0.015 to 0.020 inches in diameter. Boring or drilling of bores or passageways of this small size at the appropriate angles in the nozzle and/or gun body of prior art spray devices is a relatively expensive and difficult machining operation.
Another problem with prior art spray devices for spraying adhesive fibers is that they are not readily convertible from an adhesive bead of one diameter to a bead of another diameter. In order to change the diameter of the adhesive bead for a given application, the location of the spray jets must also be adjusted so that they remain tangent to the adhesive bead. This can require substantial modification of prior art spray devices, adding to their expense.